Positive feedback latch amplitude limiting control circuit and method of passive radio frequency identification tag

ABSTRACT

The positive feedback latch amplitude limiting control circuit and method of the passive radio frequency identification tag of the present disclosure can dynamically rectify and control the voltage between the first antenna terminal and the second antenna terminal. When the voltage between the antenna terminals is too high, the signal generating circuit turns on the discharge circuit, such that the charges at the antenna terminal are outputted to the ground, which reduces the rectified DC voltage. When the voltage between the antenna terminals is within the limited voltage, the signal generating circuit makes the discharge circuit in the turned-off state. The rectifier circuit rectifies all the charges at the antenna terminal into DC power for the load circuit, such that the current consumption is controlled to a certain extent. The discharge circuit in the present disclosure includes a MOS transistor whose gate voltage is indirectly controlled by a latch. An internal positive feedback latch mechanism of the latch makes the first control signal Si have a substantially strong pull-down driving force when being pulled to a low level, such that a control gate of the discharge path is fully turned off, which avoids a discharge state of the MOS transistor being in a sub-threshold region, and effectively reduces a leakage current in the discharge path, thereby improving communication performance.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to the field of radio frequencyidentification technology and, more particularly, relates to a positivefeedback latch amplitude limiting control circuit applied to a passiveradio frequency identification tag and a method of using the positivefeedback latch amplitude limiting control circuit to perform gradedamplitude limiting control.

BACKGROUND

A passive radio frequency identification (RFID) tag itself does notcontain a battery, and relies on electromagnetic energy sent by a cardreader to operate. Because the passive RFID tag has a simple structureand is economical and practical, the passive RFID tag has been widelyused in logistics management, asset tracking and mobile medical fields.

When operating, the passive RFID tag absorbs the electromagnetic energysent by the reader from the surrounding environment. After absorbingenergy, the passive RFID tag rectifies a part of the energy into a DCpower supply for an internal circuit of the passive RFID tag to operate.The passive RFID tag also inputs another part of the energy into aninternal modulation and demodulation circuit. The modulation anddemodulation circuit demodulates an amplitude modulated signal carriedin the energy, and sends the demodulated signal to a digital basebandportion of the passive RFID tag for processing.

In terms of energy, because a distance between the passive RFID tag andthe card reader varies, when the passive RFID tag operates, the strengthof electromagnetic field energy absorbed from the surroundingenvironment also varies. When the passive RFID tag is too close to thecard reader or the electromagnetic energy sent by the card reader is toostrong, the signal strength received by the passive RFID tag is alsosubstantially strong. Therefore, a voltage induced on the coil exceeds awithstand voltage limit of the transistor used in a rectifier module inthe chip, which causes permanent damage to the transistor, and resultsin the failure of the RFID tag.

In terms of digital communication, a transmitter coil of the card readersends out a carrier wave whose amplitude is modulated by a digitalsignal, and the passive RFID tag obtains an amplitude-modulated radiofrequency field signal through the coupling of the induction coil, anddetects and demodulates real information from the carrier wave throughthe demodulation circuit. When the passive RFID tag is too close to thecard reader or the electromagnetic energy sent by the card reader is toostrong, the demodulation circuit inside the passive RFID tag may not beable to distinguish the level of the carrier envelope signal, in otherwords, the received signal is saturated, which causes communicationfailure. This kind of failure is more likely to occur in the reader talkfirst (RTF) communication mode in which the reader first sends a commandand then waits for the response of the passive RFID tag.

To solve the above-mentioned problems where the withstand voltagereliability of the internal device in the circuit of the passive RFIDtag and the saturation of the received signal, an amplitude limitingprocessing circuit needs to be applied in the circuit of the passiveRFID tag chip, to ensure that the voltage across the antenna of thepassive RFID tag is limited to a predetermined value.

Designing an amplitude limiting circuit of the passive RFID tag faces adesign problem, in other words, under the extremely strong fieldcondition at a close distance and the extremely weak field condition ata long distance, the value of the amplitude limiting range and the pathcapability for discharging to achieve the amplitude limiting purpose aredifficult to be optimized simultaneously. On the one hand, if the valueof the amplitude limiting magnitude voltage is too high, the dischargepath will be turned on too late under the extremely strong fieldcondition, which does not effectively protect the devices with limitedinternal voltage tolerance; on the contrary, when the value of theamplitude limiting magnitude voltage is too low, the discharge path willnot be fully turned off under the extremely weak field condition, andthe leakage current will cause great energy loss. On the other hand, ifthe discharge path is designed too small, the discharge is insufficientunder the extremely strong field condition, which will also cause a riskof high voltage breakdown due to excessive voltage amplitude at theoutput terminal of the rectifier circuit; on the contrary, if thedischarge path is designed too large, the discharge path will have alarge leakage current even under the turned-off condition, which causesenergy loss and reduces communication distance. The communicationperformance of the existing passive RFID tag products in the industrymostly depends on whether the amplitude limiting circuit processingtechnology of the passive RFID is capable of achieving the optimaldesign for these two situations.

The amplitude limiting control circuit of the passive RFID tag hasreceived extensive innovative research and implementation technologydiscussions in the industry. Among them, a series of patentedtechnologies of Excelio Technology (Shenzhen) Co. Ltd. are used as themain clues. The focuses of innovation and research include twoaspects: 1) the definition, classification and judgment of the voltagelevel at the amplitude limiting point; and 2) the control mechanism ofthe discharge capacity of the amplitude limiting discharge path. First,the patented technology ZL201410009153.2, namely “Dynamic RectificationControl Circuit and Passive RFID and Dynamic Rectification ControlMethod”, adopts a method of dynamically adjusting the voltage at theamplitude limiting point, where the instantly received field energy isrectified and a mirror current formed by proportional sampling flowsthrough a fixed resistor, to generate a control voltage that controlsthe switch of the discharge path to be turned on and turned off. Inaddition, the patented technology ZL201410008854.4, namely “Amplitudelimiting Circuit and Passive Radio Frequency Tag Capable of ContinuouslyAdjusting Rectified Signal Amplitude”, adopts a bandgap referencevoltage that can be accurately defined to adjust the defined voltage ofamplitude limiting point, when the output value of the bandgap referencevoltage is the optimal value under all conditions of consideration, thedefinition of the voltage of amplitude limiting point has also beenoptimized. Moreover, another patented technology ZL201410009344.9,namely “Switch Signal-Controlled Rectification and Amplitude LimitingCircuit and Passive Radio Frequency Tag”, innovates the control signalsof the discharge paths with different discharge capabilities, andproposes that a set of switch signals are used to control thesedischarge paths, to substantially finely distinguish the dischargecapacity required for different operations under different field energyconditions. Further, a patent technology ZL201410009326.0, namely“Rectifier Amplitude Limiting Circuit and Passive Radio Frequency Tagwith Multiple Time Constants”, replaces the switch logic signal used inthe previous patent technology with analog signal with different risingedge delay and falling edge delay, to control the energy discharge basedon the time response to open the discharge path. Furthermore, a patentedtechnology ZL201410009440.3, namely “Intelligent Energy ManagementSystem and Energy Management Method for Passive Radio Frequency Tags”,puts the definition, determination method and control of the amplitudelimiting point of the passive RFID tag circuit, as well as the controlof the discharge path to the system architecture design level forplanning and design, to form the idea of intelligent RFID tag systemdesign.

The design of high-performance RFID tag circuit faces furtherchallenges, in other words, the problem of how to further improve thecommunication distance. Here, low-power circuit design technology oftenpays attention to nano-ampere (10⁻⁹ ampere) and even pico-ampere (10⁻¹²ampere) leakage current, in other words, the problem of how toeffectively reduce the problem of leakage current of the discharge pathin the turned-off state in the passive RFID circuit design.

It is known that, the deep sub-micron integrated circuit manufacturingprocess reduces the metal line width, device spacing and metal linespacing step by step, and at the same time, the turn-on thresholdvoltage of the transistor also decreases, such that the chip system canoperate at a substantially low system power supply voltage. The laws ofsemiconductor device physics show that the lower the turn-on thresholdvoltage, the lower the voltage for forming the channel and turning onthe channel conduction of the transistor. At the same time, under theturned-off condition, the leakage of the transistor device issubstantially significant. In the application of the long-distanceresponse of the passive RFID tag concerned in the present disclosure,the nano-ampere level leakage will lead to the lack of satisfactorycommunication distance, which is a bottleneck for improving theperformance of the passive RFID products.

BRIEF SUMMARY OF THE DISCLOSURE

The present disclosure provides a new technical solution for the energymanagement problem of a passive RFID tag chip. The core of the presentdisclosure includes using the circuit characteristics of the latch tomake the leakage current of the passive RFID tag chip be far less thanthe level that can be achieved by the existing technology when thedischarge path is turned off. At the same time, the energy detectionpart may smoothly turn on the discharge path under high-energy occasionsto perform amplitude limiting control. The technical solution may alsoinclude a circuit for grading the received field energy, and a methodfor using the positive feedback latch amplitude limiting control circuitto perform graded amplitude limiting control. The amplitude limitingcontrol circuit may manage the turn-on and turn-off of the dischargepath with different discharge capability at a different amplitude valueof the discharge path, such that the power consumption of the passiveRFID tag chip may be controlled, which may not only meet therequirements of the withstand voltage of the device, but also reduce thesystem energy loss in long-distance response applications.

To achieve the above purposes, the technical scheme adopted by thepresent disclosure may include a positive feedback latch amplitudelimiting control circuit of a passive radio frequency identification(RFID) tag, the circuit including: a signal generating circuit, a signalprocessing circuit and a discharge circuit, where:

the signal generating circuit is connected between an output terminal ofa rectifier circuit and ground, and is configured to: generate a firstcontrol signal S1 that varies with change of strength of a magneticfield coupled to a coil of the passive RFID tag, input the first controlsignal S1 to the signal processing circuit and the discharge circuit,and receive a logic signal S2 returned by the signal processing circuit,

the signal processing circuit is connected between the output terminalof the rectifier circuit and the ground, and is configured to: receivethe first control signal S1, latch the first control signal S1 to a highlevel or a low level according to a voltage amplitude of the firstcontrol signal S1, obtain the processed logic signal S2, and return thelogic signal S2 to the signal generating circuit, and

the discharge circuit includes a MOS transistor whose gate voltage isindirectly controlled by a latch, and source and drain of the MOStransistor are respectively connected between the output terminal of therectifier circuit and the ground, where: when the strength of themagnetic field coupled to the coil of the passive RFID tag is less thana preset value, the first control signal S1 is pulled down to the groundto maintain the discharge circuit in a turned-off state; when thestrength of the magnetic field coupled to the coil of the passive RFIDtag is greater than the preset value, the first control signal S1 is setto a high level to turn on the discharge circuit, and charges at theoutput terminal of the rectifier circuit are outputted to the ground;and an internal positive feedback latch mechanism of the latch makes thefirst control signal S1 have a substantially strong pull-down drivingforce when being pulled to a low level, such that a control gate of thedischarge path is fully turned off, which avoids a discharge state ofthe MOS transistor being in a sub-threshold region, and effectivelyreduces a leakage current in the discharge path, thereby achievingpurposes of saving system power consumption and improving communicationperformance.

The technical scheme of the present disclosure also includes a positivefeedback latch amplitude limiting control circuit of a passive radiofrequency identification (RFID) tag, the circuit including a signalgenerating circuit, a first signal processing circuit, a first dischargecircuit, a second signal processing circuit, a second signal controlcircuit and a second discharge circuit, where:

the signal generating circuit is connected between an output terminal ofa rectifier circuit and ground, and is configured to: generate a firstcontrol signal S1 that varies with change of strength of a magneticfield coupled to a coil of the passive RFID tag, input the first controlsignal S1 to the first signal processing circuit, the first dischargecircuit, and the second signal processing circuit, and receive a logicsignal S2 returned by the first signal processing circuit;

the first signal processing circuit is connected between the outputterminal of the rectifier circuit and the ground, and is configured to:receive the first control signal S1, latch the first control signal S1to a high level or a low level according to a voltage amplitude of thefirst control signal S1, obtain the processed logic signal S2, andreturn the logic signal S2 to the signal generating circuit;

the first discharge circuit includes a MOS transistor whose gate voltageis indirectly controlled by a latch, and source and drain thereof arerespectively connected between the output terminal of the rectifiercircuit and the ground, where: when the strength of the magnetic fieldcoupled to the coil of the passive RFID tag is less than a preset value,the first control signal S1 is pulled down to the ground to maintain thefirst discharge circuit in a turned-off state; when the strength of thecoupled magnetic field is greater than the preset value, the firstcontrol signal S1 is set to a high level to turn on the dischargecircuit, and charges at the output terminal of the rectifier circuit areoutputted to the ground; and an internal positive feedback latchmechanism of the latch makes the first control signal S1 have asubstantially strong pull-down driving force when being pulled to a lowlevel, such that a control gate of the discharge path is fully turnedoff, which avoids the discharge state of the MOS transistor being in asub-threshold region;

the second signal processing circuit is connected between the outputterminal of the rectifier circuit and the ground, and is configured to:receive the first control signal S1, latch the first control signal S1to the high level or the low level according to the voltage amplitude ofthe first control signal S1, obtain a processed logic signal S2′, andinput the logic signal S2′ to the second signal control circuit;

the second signal control circuit is connected between the outputterminal of the rectifier circuit and the ground, and is configured to:generate a second control signal S3 according to the high-level orlow-level of the logic signal S2′, and input the second control signalS3 to the second discharge circuit;

the second discharge circuit includes a MOS transistor whose gatevoltage is indirectly controlled by the latch, and source and drainthereof are respectively connected between the output terminal of therectifier circuit and the ground, where: when the strength of themagnetic field coupled to the coil of the passive RFID tag is less thana preset value, the first control signal S1 is pulled down to the groundto maintain the discharge circuit in the turned-off state; when thestrength of the coupled magnetic field is greater than the preset value,the first control signal S1 is set to a high level to turn on thedischarge circuit, and the charges at the output terminal of therectifier circuit are outputted to the ground; and an internal positivefeedback latch mechanism of the latch makes the first control signal S1have a substantially strong pull-down driving force when being pulled tothe low level, such that a control gate of the discharge path is fullyturned off, which avoids the discharge state of the MOS transistor beingin a sub-threshold region; and

the first signal processing circuit includes a capacitive deviceconnected to the ground, a first inverting sub-module INV1 and a secondinverting sub-module INV2, where a positive terminal of the capacitivedevice is connected to a latch flip-flop with positive feedbackcharacteristic formed by connecting an output terminal of the firstinverting sub-module INV1 and an input terminal of the second invertingsub-module INV2, and connecting an input terminal of the first invertingsub-module INV1 and an output terminal of the second invertingsub-module INV2; and the second signal processing circuit includes adetector circuit formed by at least one unidirectional conducting devicewith a threshold voltage and a capacitive device, a third invertingsub-module INV3 and a fourth inverting sub-module INV4, where an outputterminal of the detector circuit is connected to a latch flip-flop withpositive feedback characteristic formed by connecting an output terminalof the third inverting sub-module INV3 and an input terminal of thefourth inverting sub-module INV4, and connecting an input terminal ofthe first inverting sub-module INV1 and an output terminal of the secondinverting sub-module INV2.

The present disclosure also provides a method of using the abovedisclosed positive feedback latch amplitude limiting control circuit toperform a graded amplitude limiting control, the method includingfollowing steps:

S1: a resonance circuit is coupled with an external magnetic field togenerate an alternating current and inputs the alternating current tothe rectifier circuit, and the rectifier circuit rectifies thealternating current into a direct current and outputs the direct currentto each circuit module including the signal generating circuit, thesignal processing circuit and the discharge circuit;

S2: the signal generating circuit generates the first control signal S1that varies with the change of the strength of the magnetic fieldcoupled to the coil of the passive RFID tag, and inputs the firstcontrol signal S1 to the signal processing circuit, where: when thevoltage value of the first control signal S1 is too low to drive theinverting sub-module in the signal processing circuit to be turned on,the logic signal S2 outputted by the signal processing circuit is a highlevel “1”; when the voltage value of the first control signal S1gradually increases to the turn-on voltage of the inverting sub-module,the logic signal S2 outputted by the signal processing circuit is a lowlevel “0”; and the signal processing circuit returns the logic signal S2of the high level “1” or the low level “0” to the signal generatingcircuit; and

S3: when the inputted logic signal S2 is the high level “1”, the firstN-type MOS transistor in the signal generating circuit is turned on, andthe second resistive device is short-circuited; because the resistancevalue of the first resistive device is far smaller than the resistancevalue of the second resistive device, the first control signal S1 israpidly pulled down to a low level signal, such that the second N-typeMOS transistor in the discharge circuit is turned off, and the dischargepath maintains the turned-off state; when the inputted logic signal S2is the low level “0”, the first N-type MOS transistor is turned off, andthe first control signal S1 maintains a high-level signal, such that thesecond N-type MOS transistor in the discharge circuit is turned on, thedischarge path is switched from the turned-off state to the turned-onstate, and the charges at an antenna terminal are outputted to theground.

The graded control method also includes following steps:

S1: a resonance circuit is coupled with an external magnetic field togenerate an alternating current and inputs the alternating current tothe rectifier circuit, and the rectifier circuit rectifies thealternating current into a direct current and outputs the direct currentto each circuit module including the signal generating circuit, thefirst signal processing circuit, the first discharge circuit, the secondsignal processing circuit, the second signal control circuit and thesecond discharge circuit;

S2: the signal generating circuit generates the first control signal S1that varies with the change of the strength of the magnetic fieldcoupled to the coil of the passive RFID tag, and inputs the firstcontrol signal S1 to the first signal processing circuit and the secondsignal processing circuit, where: when the voltage value of the firstcontrol signal S1 is too low to drive the inverting sub-module in thefirst signal processing circuit to be turned on, the logic signal S2outputted by the first signal processing circuit is a high level “1”;when the voltage value of the first control signal S1 graduallyincreases to the turn-on voltage of the inverting sub-module, the logicsignal S2 outputted by the first signal processing circuit is a lowlevel “0”; the first signal processing circuit returns the logic signalS2 of the high level “1” or the low level “0” to the signal generatingcircuit; and when the voltage value of the first control signal S1 islower than the sum of the threshold voltage of the unidirectionalconducting device with a threshold voltage and the turn-on voltage ofthe inverting sub-module in the second signal processing circuit, thelogic signal S2′ outputted by the second signal processing circuit isthe high level “1”; when the voltage value of the first control signalS1 gradually increases to the sum of the threshold voltage of thethreshold device and the turn-on voltage of the inverting sub-module inthe second signal processing circuit, the logic signal S2′ outputted bythe second signal processing circuit is the low level “0”; and thesecond signal processing circuit inputs the above logic signal S2′ ofthe high level “1” or the low level “0” to the second signal controlcircuit; and

S3: when the inputted logic signal S2 is the high level “1”, the firstN-type MOS transistor in the signal generating circuit is turned on, andthe second resistive device is short-circuited; because the resistancevalue of the first resistive device is far smaller than the resistancevalue of the second resistive device, the first control signal S1 israpidly pulled down to a low level signal, such that the second N-typeMOS transistor in the first discharge circuit is turned off, and thefirst discharge path maintains the turned-off state; when the inputtedlogic signal S2 is the low level “0”, the first N-type MOS transistor isturned off, and the first control signal S1 maintains a high-levelsignal, such that the second N-type MOS transistor is turned on, thefirst discharge path is switched from the turned-off state to theturned-on state, and the charges at an antenna terminal are outputted tothe ground; when the inputted logic signal S2′ is the high level “1”,the third N-type MOS transistor in the second signal control circuit isturned on, and the fourth resistive device is short-circuited; becausethe resistance value of the third resistive device is far smaller thanthe resistance value of the fourth resistive device, the second controlsignal S3 is rapidly pulled down to a low level signal, such that thefourth N-type MOS transistor in the second discharge circuit is turnedoff, and the second discharge path maintains the turned-off state; whenthe inputted logic signal S2′ is the low level “0”, the third N-type MOStransistor is turned off, and the third control signal S3 maintains ahigh-level signal, such that the fourth N-type MOS transistor is turnedon, the second discharge path is switched from the turned-off state tothe turned-on state, and the charges at the antenna terminal areoutputted to the ground.

The positive feedback latch amplitude limiting control circuit andmethod of the passive radio frequency identification tag of the presentdisclosure may dynamically rectify and control the voltage between thefirst antenna terminal and the second antenna terminal. When the voltagebetween the antenna terminals is too high, the signal generating circuitmay generate a high-level signal, and after being processed, a logic lowsignal may be generated, and the generated high-level signal may beinputted into the discharge circuit to turn on the discharge circuit,such that the charges at the antenna terminal may be outputted to theground, which may reduce the amount of charges at the antenna terminaland may reduce the rectified DC voltage. When the voltage between theantenna terminals is within the limited voltage, the signal generatingcircuit may make the signal processing circuit generate a logic highsignal, and the correspondingly generated low-level signal may make thedischarge circuit in the turned-off state. The rectifier circuit mayrectify all the charges at the antenna terminal into DC power for theload circuit, such that the current consumption may be controlled to acertain extent and the system energy loss may be reduced.

In addition, the discharge circuit in the present disclosure may includethe MOS transistor whose gate voltage is indirectly controlled by thelatch, and the internal positive feedback latch mechanism of the latchmay make the first control signal S1 have a substantially strongpull-down driving force when being pulled to a low level, such that thecontrol gate of the discharge path may be fully turned off, which mayavoid the discharge state of the MOS transistor being in a sub-thresholdregion, may effectively reduce the leakage current in the dischargepath, may save system power consumption, may improve communicationperformance, and may play a role that cannot be achieved by the controlvoltage outputted by the ordinary logic circuit. The circuit of thepresent disclosure may have a simple structure, may be easy to realize,and may basically have no static power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a circuit structure consistentwith Embodiment a of the present disclosure;

FIG. 2 illustrates a schematic diagram of a signal generating circuitand a discharge circuit consistent with Embodiment 1 of the presentdisclosure;

FIG. 3 illustrates a schematic diagram of a first implementationstructure of a signal processing circuit consistent with Embodiment 1 ofthe present disclosure;

FIG. 4 illustrates a schematic diagram of a second implementationstructure of a signal processing circuit consistent with Embodiment 1 ofthe present disclosure;

FIG. 5 illustrates a schematic diagram of a circuit structure consistentwith Embodiment 2 of the present disclosure;

FIG. 6 illustrates a schematic diagram of a first signal processingcircuit consistent with Embodiment 2 of the present disclosure;

FIG. 7 illustrates a schematic diagram of a second signal processingcircuit and a second signal control circuit consistent with Embodiment 2of the present disclosure; and

FIG. 8 illustrates a schematic diagram of a second discharge circuitconsistent with Embodiment 2 of the present disclosure.

DETAILED DESCRIPTION

The technical solutions in the disclosed embodiments of the presentdisclosure will be clearly and fully described below with reference tothe accompanying drawings in the embodiments of the present disclosure.Obviously, the described embodiments are only a part of the embodimentsof the present disclosure, but are not all of the embodiments. Based onthe disclosed embodiments of the present disclosure, all otherembodiments obtained by those of ordinary skill in the art withoutcreative efforts shall fall within the protection scope of the presentdisclosure.

Embodiment 1

Referring to FIG. 1 , a positive feedback latch amplitude limitingcontrol circuit of a passive radio frequency identification tag in thepresent disclosure may include a resonance circuit, a rectifier circuitand an energy storage capacitor C2. The circuit may also include asignal generating circuit, a signal processing circuit and a dischargecircuit.

The signal generating circuit may be connected between an outputterminal of the rectifier circuit and the ground, may be configured togenerate a first control signal S1 that varies with the change of thestrength of magnetic field coupled to a coil of the passive RFID tag, toinput the first control signal S1 to the signal processing circuit andthe discharge circuit, and to receive a logic signal S2 returned by thesignal processing circuit.

The signal processing circuit may be connected between the outputterminal of the rectifier circuit and the ground, may be configured toreceive the first control signal S1, to latch the first control signalS1 to a high level or a low level according to a voltage amplitude ofthe first control signal S1, to obtain the processed logic signal S2,and to return the logic signal S2 to the signal generating circuit.

The discharge circuit may include a MOS transistor whose gate voltage isindirectly controlled by a latch, and source and drain of the MOStransistor may be respectively connected between the output terminal ofthe rectifier circuit and the ground. When the strength of magneticfield coupled to the coil of the passive RFID tag is less than a presetvalue, the first control signal S1 may be pulled down to the ground tomaintain the discharge circuit in a turned-off state. When the strengthof magnetic field coupled to the coil of the passive RFID tag is greaterthan the preset value, the first control signal S1 may be set to a highlevel to turn on the discharge circuit, the charges at the outputterminal of the rectifier circuit may be outputted to the ground. Theinternal positive feedback latch mechanism of the latch may make thefirst control signal S1 have a substantially strong pull-down drivingforce when being pulled to a low level, such that the control gate ofthe discharge path may be fully turned off, which may avoid thedischarge state of the MOS transistor being in a sub-threshold region,and may effectively reduce the leakage current in the discharge path,thereby achieving the purposes of saving system power consumption andimproving communication performance.

The positive feedback latch amplitude limiting control circuit of thepassive radio frequency identification tag in the present disclosure maydynamically rectify and control the voltage between a first antennaterminal and a second antenna terminal. When the voltage between theantenna terminals is too high, the signal generating circuit maygenerate a high-level signal, and after being processed, a logic lowsignal may be generated, and the generated high-level signal may beinputted into the discharge circuit to turn on the discharge circuit,such that the charges at the antenna terminal may be outputted to theground, which may reduce the amount of charges at the antenna terminaland may reduce the rectified DC voltage. When the voltage between theantenna terminals is within the limited voltage, the signal generatingcircuit may make the signal processing circuit generate a logic highsignal, and the correspondingly generated low-level signal may make thedischarge circuit in the turned-off state. The rectifier circuit mayrectify all the charges at the antenna terminal into DC power for theload circuit, such that the current consumption may be controlled to acertain extent and the system energy loss may be reduced.

Moreover, in the present disclosure, the turn-on and turn-off of thedischarge path may be achieved by the output voltage of the latch. Thelatch circuit may have various forms of implementation, and the corefeature of the latch circuit may include that the internal structurethereof may have a latch mechanism with positive feedback gain.Therefore, when being at a high level or at a low level, the outputvoltage of the latch circuit may have a stronger pull-up or pull-downcapability than the output voltage of an ordinary logic circuit, andsuch feature may often be used for information preservation inelectronic systems. In the present disclosure, the latch may be used tocontrol the turn-on and turn-off of the discharge path of the passiveRFID tag chip, which may play a role that cannot be achieved by thecontrol voltage outputted by the ordinary logic circuit. For example,when the output voltage of the latch is pulled down to the ground, thepull-down degree may be much greater than the pull-down degree of theoutput node of the ordinary logic circuit, such that the MOS switchcontrolled by the pull-down signal may be fully turned off and may nothave leakage current due to the perturbation of the ground noise. Suchfeature may be especially significant in deep sub-micron integratedcircuit manufacturing process. In these process nodes, due to thepursuit of low power supply voltage and high-speed switchingcharacteristics, the turn-on threshold voltage of the MOS transistor maybe made substantially low, regardless of whether the channel is turnedon or not, such that a considerable leakage current may exist in thesub-threshold region, and such leakage current is not conducive to theapplication of the passive RFID tag.

To examine the effect of turning off the discharge path of suchstructure from the perspective of pull-down capability alone, comparedwith the patented technology ZL201410009153.2, namely “DynamicRectification Control Circuit and Passive RFID and Dynamic RectificationControl Method”, the voltage output of the latch in the presentdisclosure may have a significantly enhanced pull-down capability thanthe voltage output generated on a resistor node. Compared with thepatented technology ZL201410008854.4, namely “Amplitude limiting Circuitand Passive Radio Frequency Tag Capable of Continuously AdjustingRectified Signal Amplitude”, the determination of the input voltage bythe latch may be determined by the latch inversion operation triggeredby the turn-on voltage of a threshold device. Compared with anotherpatented technology ZL201410009344.9, namely “Switch Signal-ControlledRectification and Amplitude Limiting Circuit and Passive Radio FrequencyTag”, the pull-down capability of the output voltage of the latch may beaffected by the positive feedback gain factors, which may be strongerthan the pull-down capability of the voltage outputted by any otherlogic switch circuit. Compared with the patented technologyZL201410009326.0, namely “Rectifier Amplitude Limiting Circuit andPassive Radio Frequency Tag with Multiple Time Constants”, the outputvoltage of the latch may be a logic signal with a very short timeconstant, rather than an analog signal with variable amplitude limitingcontrol over time constant.

FIG. 2 illustrates a schematic diagram of a signal generating circuitand a discharge circuit consistent with Embodiment 1 of the presentdisclosure. The signal generating circuit may include a first currentsource I1, a first resistive device R1 and a second resistive device R2connected in series between the power supply terminal and the ground. Afirst control signal S1 may be outputted from a node between the firstcurrent source I1 and the first resistive device R1. A gate of a firstN-type MOS transistor NM1 may be connected to an output terminal of thesignal processing circuit, a drain of the first N-type MOS transistorNM1 may be connected to a node between the first resistive device R1 andthe second resistive device R2, and a source of the first N-type MOStransistor NM1 may be connected to a node between the second resistivedevice R2 and the ground. A resistance value of the first resistivedevice R1 may be much smaller than a resistance value of the secondresistive device R2.

The discharge circuit may include a second N-type MOS transistor NM2connected between an output terminal of the rectifier circuit and theground. A drain of the second N-type MOS transistor NM2 may be connectedto the power supply terminal, a source of the second N-type MOStransistor NM2 may be grounded, and a gate of the second N-type MOStransistor NM2 may be connected to the output terminal of the firstcurrent source I1. The N-type MOS transistor NM2 may be configured toturn on the discharge circuit under the control of the first controlsignal S1 when the strength of magnetic field of the electromagneticfield is too strong, and the charges at the output terminal of therectifier circuit may be outputted to the ground. Because the gatesignal of the discharge circuit, in other words, the first controlsignal S1, is connected to a terminal of the latch, according to theworking principle of the latch, the inherent positive feedback mechanismof the latch may make the first control signal S1 have a substantiallystrong pull-down driving force when being pulled to a low level, suchthat the control gate of the discharge path may be fully turned off, toavoid the discharge state of the MOS transistor being in a sub-thresholdregion, thereby effectively reducing the leakage current in thedischarge path, saving system power consumption, and improvingcommunication performance.

FIG. 3 illustrates a schematic diagram of a first implementationstructure of a signal processing circuit consistent with Embodiment 1 ofthe present disclosure. The signal processing circuit may include acapacitive device connected to the ground, such as a third capacitor C3,a first inverting sub-module INV1 and a second inverting sub-moduleINV2. A positive terminal of the capacitive device may be connected to alatch flip-flop with positive feedback characteristic formed byconnecting an output terminal of the first inverting sub-module INV1 andan input terminal of the second inverting sub-module INV2, andconnecting an input terminal of the first inverting sub-module INV1 andan output terminal of the second inverting sub-module INV2. The signalprocessing circuit may convert an analog signal to a digital signal, andmay be configured to rapidly convert the first control signal S1 into ahigh-level or low-level logic signal according to an analog voltageamplitude of the first control signal S1, to obtain a processed logicsignal S2, and to return the logic signal S2 to the signal generatingcircuit.

FIG. 4 illustrates a schematic diagram of a second implementationstructure of a signal processing circuit consistent with Embodiment 1 ofthe present disclosure. The signal processing circuit may include adetector circuit formed by at least one unidirectional conducting devicewith a threshold voltage, such as a diode D1, and a capacitive device,such as the third capacitor C3, and the first inverting sub-module INV1and the second inverting sub-module INV2. An output terminal of thedetector circuit may be connected to the latch flip-flop with positivefeedback characteristic formed by connecting an output terminal of thefirst inverting sub-module INV1 and an input terminal of the secondinverting sub-module INV2, and connecting an input terminal of the firstinverting sub-module INV1 and an output terminal of the second invertingsub-module INV2. The signal processing circuit may convert an analogsignal to a digital signal, and may be configured to rapidly convert thefirst control signal S1 into a high-level or low-level logic signalaccording to an analog voltage amplitude of the first control signal S1,to obtain the processed logic signal S2, and to return the logic signalS2 to the signal generating circuit.

In the first and second implementation structures of the above-disclosedsignal processing circuit, the inverting sub-module may adopt theinverting sub-module circuit with simplest form formed by a PMOStransistor and an NMOS transistor connected in series. Similarly, theinverting sub-module may also adopt a logic circuit with equivalentlogic inversion function and formed by AND gate, NAND gate, OR gate, NORgate, and XOR gate, or an analog circuit with equivalent logic inversionfunction and formed by analog differential amplifier. The latch may alsobe extended to a latch structure with positive feedback characteristicsand positive feedback connection in general form, such as a latchstructure formed by driving a positive feedback load with a differentialpair transistor.

The difference between the second implementation structure and the firstimplementation structure may include that the unidirectional conductingdevice with a threshold voltage may be added in the front end of thelatch flip-flop. Therefore, on the basis of the first implementationstructure, the conduction voltage value of the first control signal S1may increase. In the first implementation structure, when the firstcontrol signal S1 rises to a turn-on voltage of the invertingsub-module, the logic signal S2 outputted by the signal processingcircuit may be a low level “0”. In the second implementation structure,only when the voltage value of the first control signal S1 rises to asum of the threshold voltage of the threshold device and the turn-onvoltage of the inverting sub-module, the logic signal S2 outputted bythe signal processing circuit may be a low level “0”. Therefore,different quantities of threshold devices may be selected and connectedaccording to circuit design requirements, product usage requirements andother parameters (the quantity of threshold devices may not only bedetermined during circuit design, but also be capable of being changedlater by laser trimming process).

According to the above discussion, when the field strength is toostrong, the first control signal S1 may be at a high level, the logicsignal S2 may be at a low level, the first N-type MOS transistor NM1 mayremain in a turned-off state, and the voltage value V_(S1)=I₁*(R₁+R₂) ofthe first control signal S1 may be at a high level, such that the secondN-type MOS transistor NM2 may be turned on for discharging. When thefirst control signal S1 is at a low level, the logic signal S2 may be ata high level, the first N type MOS transistor NM1 may be turned on toshort-circuit the second resistive device R2. In view of this, thevoltage value of the first control signal S1 may be V_(S1)′=I₁*R₁,because the resistance value of the first resistive device R1 is muchsmaller than the resistance value of the second resistive device R2,V_(S1)′ may be much smaller than V_(S1) and may be a low-level signal,such that the second N-type MOS transistor NM2 may be in a turned-offstate to stop discharging.

A working principle of the above-disclosed graded amplitude limitingcontrol circuit of the passive RFID tag may include following. The LCresonance circuit may collect the energy from the electromagnetic fieldemitted by the card reader, and the energy may be rectified by therectifier circuit to supply power to the system internal circuit. Thesignal generating circuit may generate the first control signal S1 thatvaries with the change of the strength of magnetic field of theelectromagnetic field, which may be inputted into the signal processingcircuit and the discharge circuit. The stronger the strength of magneticfield, the higher the first control signal S1, and vice versa. When thefirst control signal S1 is substantially high, the signal processingcircuit may output a low-level logic signal S2 to return to the signalgenerating circuit, such that the first N-type MOS transistor NM1 in thesignal generating circuit may remain in the turned-off state, and thefirst control signal S1 may maintain a high potential. Therefore, thesecond N-type MOS transistor NM2 in the discharge circuit may be turnedon, the discharge circuit may be in a discharge state, and the chargesat the antenna terminal may be outputted to the ground. When theelectromagnetic field weakens to a certain extent, the first controlsignal S1 may be reduced to a certain extent. In view of this, thesignal processed by the signal processing circuit may be inverted, thelogic signal S2 may jump from a low level to a high level, the firstN-type MOS transistor NM1 may be turned on, the second resistor R2 maybe short-circuited by the first N-type MOS transistor NM1, and the firstcontrol signal S1 may be pulled down. In view of this, the second N-typeMOS transistor NM2 may be in a turned-off state to stop discharging, andthe system current consumption may be reduced.

Embodiment 2

FIG. 5 illustrates a schematic diagram of a circuit structure consistentwith Embodiment 2 of the present disclosure. In one embodiment, thepositive feedback latch amplitude limiting control circuit may include aresonance circuit, a rectifier circuit and an energy storage capacitorC2. The positive feedback latch amplitude limiting control circuit mayfurther include the signal generating circuit, a first signal processingcircuit, a first discharge circuit, a second signal processing circuit,a second signal control circuit and a second discharge circuit.

The signal generating circuit may be connected between the outputterminal of the rectifier circuit and the ground, and may be configuredto generate the first control signal S1 that varies with the change ofthe strength of the magnetic field coupled to the coil of the passiveRFID tag, and to input the first control signal S1 to the first signalprocessing circuit, the first discharge circuit, and the second signalprocessing circuit, and to receive the logic signal S2 returned by thefirst signal processing circuit.

The first signal processing circuit may be connected between the outputterminal of the rectifier circuit and the ground, and may be configuredto receive the first control signal S1, to latch the first controlsignal S1 to a high level or a low level according to the voltageamplitude of the first control signal S1, to obtain the processed logicsignal S2, and to return the logic signal S2 to the signal generatingcircuit.

The first discharge circuit may include a MOS transistor whose gatevoltage is indirectly controlled by the latch, and source and drain ofthe MOS transistor may be respectively connected between the outputterminal of the rectifier circuit and the ground. When the strength ofmagnetic field coupled to the coil of the passive RFID tag is less thana preset value, the first control signal S1 may be pulled down to theground to maintain the discharge circuit in a turned-off state. When thestrength of magnetic field coupled to the coil of the passive RFID tagis greater than the preset value, the first control signal S1 may be setto a high level to turn on the discharge circuit, the charges at theoutput terminal of the rectifier circuit may be outputted to the ground.The internal positive feedback latch mechanism of the latch may make thefirst control signal S1 have a substantially strong pull-down drivingforce when being pulled to a low level, such that the control gate ofthe discharge path may be fully turned off, which may avoid thedischarge state of the MOS transistor being in a sub-threshold region.

The second signal processing circuit may be connected between the outputterminal of the rectifier circuit and the ground, and may be configuredto receive the first control signal S1, to latch the first controlsignal S1 to a high level or a low level according to the voltageamplitude of the first control signal S1, to obtain a processed logicsignal S2′, and to input the logic signal S2′ to the second signalcontrol circuit.

The second signal control circuit may be connected between the outputterminal of the rectifier circuit and the ground, and may be configuredto generate a second control signal S3 according to the high-level orlow-level of the logic signal S2′, and to input the second controlsignal S3 to the second discharge circuit.

The second discharge circuit may include a MOS transistor whose gatevoltage is indirectly controlled by the latch, and source and drain ofthe MOS transistor may be respectively connected between the outputterminal of the rectifier circuit and the ground. When the strength ofmagnetic field coupled to the coil of the passive RFID tag is less thana preset value, the first control signal S1 may be pulled down to theground to maintain the discharge circuit in a turned-off state. When thestrength of magnetic field coupled to the coil of the passive RFID tagis greater than the preset value, the first control signal S1 may be setto a high level to turn on the discharge circuit, the charges at theoutput terminal of the rectifier circuit may be outputted to the ground.The internal positive feedback latch mechanism of the latch may make thefirst control signal S1 have a substantially strong pull-down drivingforce when being pulled to a low level, such that the control gate ofthe discharge path may be fully turned off, which may avoid thedischarge state of the MOS transistor being in a sub-threshold region.

The first signal processing circuit may include a capacitive deviceconnected to the ground, the first inverting sub-module INV1 and thesecond inverting sub-module INV2. The positive terminal of thecapacitive device may be connected to the latch flip-flop with positivefeedback characteristic formed by connecting an output terminal of thefirst inverting sub-module INV1 and an input terminal of the secondinverting sub-module INV2, and connecting an input terminal of the firstinverting sub-module INV1 and an output terminal of the second invertingsub-module INV2. The second signal processing circuit may include adetector circuit formed by at least one unidirectional conducting devicewith a threshold voltage and a capacitive device, a third invertingsub-module INV3 and a fourth inverting sub-module INV4. An outputterminal of the detector circuit may be connected to the latch flip-flopwith positive feedback characteristic formed by connecting an outputterminal of the third inverting sub-module INV3 and an input terminal ofthe fourth inverting sub-module INV4, and connecting an input terminalof the first inverting sub-module INV1 and an output terminal of thesecond inverting sub-module INV2.

In one embodiment, the signal generating circuit, the first signalprocessing circuit and the first discharge circuit may adopt the samestructure as the signal generating circuit, the signal processingcircuit and the discharge circuit described in Embodiment 1, which maynot be repeated herein. It should be particularly noted that the signalprocessing circuit may adopt the first implementation structure inEmbodiment 1, in other words, a threshold-free device structure, asshown in FIG. 6 .

In present technical solution, the graded amplitude limiting may beachieved by parallel connecting more than one signal processing circuit,namely the first signal processing circuit and the second signalprocessing circuit, and parallel processing the first control signal S1generated by the signal generating circuit and representing the strengthof the coupling field. Because the second signal processing circuit hasat least one more unidirectional conducting device with a thresholdvoltage than the first signal processing circuit, the second signalprocessing circuit may have a higher flip voltage of the latch than thefirst signal processing circuit, in other words, a higher amplitudelimiting voltage may be required to turn on the second dischargecircuit. In fact, the flip voltage of the latch in the first signalprocessing circuit may be the flip voltage of the inverting sub-modulein the latch. If the inverting sub-module is simply composed of PMOS andNMOS connected in series, the flip voltage may be a threshold voltage ofthe NMOS transistor. Due to the existence of a unidirectional conductingdevice with a threshold voltage in the second signal processing circuit,the flip voltage may become a sum of the threshold voltage of the addedthreshold device and the flip voltage of the inverting sub-modulecorresponding to the latch in the second signal processing circuit.

FIG. 7 illustrates a schematic diagram of the second signal processingcircuit and the second signal control circuit consistent with Embodiment2 of the present disclosure. The second signal processing circuit mayinclude a detector circuit formed by at least one unidirectionalconducting device with a threshold voltage, such as a diode D2, and acapacitive device, such as a fourth capacitor C4, the third invertingsub-module INV3 and the fourth inverting sub-module INV4. The outputterminal of the detector circuit may be connected to the latch flip-flopwith positive feedback characteristic formed by connecting an outputterminal of the third inverting sub-module INV3 and an input terminal ofthe fourth inverting sub-module INV4, and connecting an input terminalof the third inverting sub-module INV3 and an output terminal of thefourth inverting sub-module INV4. The second signal processing circuitmay convert an analog signal to a digital signal, and may be configuredto rapidly convert the first control signal S1 into a high-level orlow-level logic signal according to the analog voltage amplitude of thefirst control signal S1, to obtain the processed logic signal S2′, andto return the logic signal S2′ to the signal generating circuit.

The second signal control circuit may include a second current source12, a third resistive device R3 and a fourth resistive device R4connected in series between the power supply terminal and the ground. Asecond control signal S3 may be outputted from a node between the secondcurrent source 12 and the third resistive device R3. A gate of a switchdevice, in other words, a third N-type MOS transistor NM3, may beconnected to an output terminal of the second signal processing circuit,a drain of the third N-type MOS transistor NM3 may be connected to anode between the third resistive device R3 and the fourth resistivedevice R4, and a source of the third N-type MOS transistor NM3 may beconnected to a node between the fourth resistive device R4 and theground. A resistance value of the third resistive device R3 may be muchsmaller than a resistance value of the fourth resistive device R4.

Because the input signal S1 of the second signal processing circuitstill has a little ripple fluctuation, the signal S1 may become ananalog voltage with a smooth amplitude after passing through thedetector circuit. When the voltage value of the signal S1 is greaterthan the turn-on voltage of the latch flip-flop, the latch flip-flop mayquickly flip to generate the logic signal S2′. The logic signal S2′ maycontrol the turn-on and turn-off of the switch device NM3, therebydetermining whether the resistive device R4 is short-circuited, in otherwords, determining the amplitude of the signal S3.

FIG. 8 illustrates a schematic diagram of a second discharge circuitconsistent with Embodiment 2 of the present disclosure. The seconddischarge circuit may include a fourth N-type MOS transistor NM4connected between the output terminal of the rectifier circuit and theground. The drain of the fourth N-type MOS transistor NM4 may beconnected to the power supply terminal, the source of the fourth N-typeMOS transistor NM4 may be connected to the ground, and the gate of thefourth N-type MOS transistor NM4 may be connected to the output terminalof the second signal control circuit. Under the control of the secondcontrol signal S3, when the strength of magnetic field of theelectromagnetic field is too strong, the second discharge circuit may beturned on, and the charges at the output terminal of the rectifiercircuit may be outputted to the ground. Because the gate signal of thedischarge circuit, in other words, the first control signal S1 is oneterminal of the latch, according to the working principle of the latch,the internal positive feedback latch mechanism of the latch may make thefirst control signal S1 have a substantially strong pull-down drivingforce when being pulled to a low level, such that the control gate ofthe discharge path may be fully turned off, which may avoid thedischarge state of the MOS transistor NM4 being in a sub-thresholdregion, and may effectively reduce the leakage current in the dischargepath, thereby saving system power consumption and improvingcommunication performance.

In one embodiment, a structure in which two discharge paths areconnected in parallel may be adopted. Because the first discharge pathdoes not include a threshold device, when the first control signal S1 ishigher than the turn-on voltage of the first inverting sub-module andthe second inverting sub-module, the logic signal S2 may output a lowlevel “0”, and the discharge circuit in the unit may begin to discharge.In view of this, in the second discharge path, due to the blockingeffect of the threshold device, the first control signal S1 may be lowerthan the sum of the threshold voltage and the turn-on voltage of thethird inverting sub-module and the fourth inverting sub-module, thelogic signal S2′ may maintain a high level “1”, such that the dischargepath may be in a turned-off state. Merely when the first control signalS1 continues to rise to a value greater than the sum of the thresholdvoltage of the threshold device and the turn-on voltage of the invertingsub-module, the second discharge path may be turned on to discharge.

Therefore, in the circuit design, parameters such as the impedance valueof the resistive device, the threshold value of the threshold device,and the width-to-length ratio of the N-type MOS transistor in the twodischarge paths connected in parallel may be optimized, such that thetwo discharge paths may have different amplitude limiting leakage pointsand different current leakage capabilities, thus playing the role ofgraded amplitude limiting. In the circuit design and use process, theparameters of each discharge path device may be adjusted according tothe use environment of the circuit and product requirements, to achievethe optimal energy harvesting and discharge scheme, thereby maximizingthe product performance.

The present disclosure also provides a method of using the abovedisclosed positive feedback latch amplitude limiting control circuit toperform graded amplitude limiting control. The method may includefollowing steps.

S1: A resonance circuit may be coupled with an external magnetic fieldto generate an alternating current and may input the alternating currentto the rectifier circuit, the rectifier circuit may rectify thealternating current into a direct current and may output the directcurrent to each circuit module including the signal generating circuit,the signal processing circuit and the discharge circuit.

S2: The signal generating circuit may generate the first control signalS1 that varies with the change of the strength of magnetic field coupledto the coil of the passive RFID tag, and the first control signal S1 maybe inputted to the signal processing circuit. When the voltage value ofthe first control signal S1 is too low to drive the inverting sub-modulein the signal processing circuit to be turned on, the logic signal S2outputted by the signal processing circuit may be a high level “1”. Whenthe voltage value of the first control signal S1 gradually increases tothe turn-on voltage of the inverting sub-module, the logic signal S2outputted by the signal processing circuit may be a low level “0”. Thesignal processing circuit may return the logic signal S2 of the highlevel “1” or the low level “0” to the signal generating circuit.

S3: When the inputted logic signal S2 is the high level “1”, the firstN-type MOS transistor in the signal generating circuit may be turned on,the second resistive device may be short-circuited. Because theresistance value of the first resistive device is far smaller than theresistance value of the second resistive device, the first controlsignal S1 may be rapidly pulled down to a low level signal, such thatthe second N-type MOS transistor in the discharge circuit may be turnedoff, and the discharge path may maintain the turned-off state. When theinputted logic signal S2 is the low level “0”, the first N-type MOStransistor may be turned off, and the first control signal S1 maymaintain a high-level signal, such that the second N-type MOS transistorin the discharge circuit may be turned on, the discharge path may beswitched from the turned-off state to the turned-on state, and thecharges at the antenna terminal may be outputted to the ground.

When the positive feedback latch amplitude limiting control circuitadopts the structure described in Embodiment 2 of the presentdisclosure, the graded amplitude limiting control method may includefollowing steps.

S1: A resonance circuit may be coupled with an external magnetic fieldto generate an alternating current and may input the alternating currentto the rectifier circuit, the rectifier circuit may rectify thealternating current into a direct current and may output the directcurrent to each circuit module including the signal generating circuit,the first signal processing circuit, the first discharge circuit, thesecond signal processing circuit, the second signal control circuit andthe second discharge circuit.

S2: The signal generating circuit may generate the first control signalS1 that varies with the change of the strength of magnetic field coupledto the coil of the passive RFID tag, and the first control signal S1 maybe inputted to the first signal processing circuit and the second signalprocessing circuit, respectively. When the voltage value of the firstcontrol signal S1 is too low to drive the inverting sub-module in thefirst signal processing circuit to be turned on, the logic signal S2outputted by the first signal processing circuit may be a high level“1”. When the voltage value of the first control signal S1 graduallyincreases to the turn-on voltage of the inverting sub-module in thefirst signal processing circuit, the logic signal S2 outputted by thefirst signal processing circuit may be a low level “0”. The first signalprocessing circuit may return the above logic signal S2 of the highlevel “1” or the low level “0” to the signal generating circuit. Whenthe voltage value of the first control signal S1 is lower than the sumof the threshold voltage of the unidirectional conducting device with athreshold voltage and the turn-on voltage of the inverting sub-module inthe second signal processing circuit, the logic signal S2′ outputted bythe second signal processing circuit may be a high level “1”. When thevoltage value of the first control signal S1 gradually increases to thesum of the threshold voltage of the threshold device and the turn-onvoltage of the inverting sub-module in the second signal processingcircuit, the logic signal S2′ outputted by the second signal processingcircuit may be a low level “0”. The second signal processing circuit mayinput the above logic signal S2′ of the high level “1” or the low level“0” to the second signal control circuit.

S3: When the inputted logic signal S2 is the high level “1”, the firstN-type MOS transistor in the signal generating circuit may be turned on,the second resistive device may be short-circuited. Because theresistance value of the first resistive device is far smaller than theresistance value of the second resistive device, the first controlsignal S1 may be rapidly pulled down to a low level signal, such thatthe second N-type MOS transistor in the first discharge circuit may beturned off, and the first discharge path may maintain the turned-offstate. When the inputted logic signal S2 is the low level “0”, the firstN-type MOS transistor may be turned off, and the first control signal S1may maintain a high-level signal, such that the second N-type MOStransistor may be turned on, the first discharge path may be switchedfrom the turned-off state to the turned-on state, and the charges at theantenna terminal may be outputted to the ground. When the inputted logicsignal S2′ is the high level “1”, the third N-type MOS transistor in thesecond signal control circuit may be turned on, and the fourth resistivedevice may be short-circuited. Because the resistance value of the thirdresistive device is far smaller than the resistance value of the fourthresistive device, the second control signal S3 may be rapidly pulleddown to a low level signal, such that the fourth N-type MOS transistorin the second discharge circuit may be turned off, and the seconddischarge path may maintain the turned-off state. When the inputtedlogic signal S2′ is the low level “0”, the third N-type MOS transistormay be turned off, and the third control signal S3 may maintain ahigh-level signal, such that the fourth N-type MOS transistor may beturned on, the second discharge path may be switched from the turned-offstate to the turned-on state, and the charges at the antenna terminalmay be outputted to the ground.

In the above-disclosed graded amplitude limiting control method, whenthe field strength changes from weak to strong (in other words, the chipapproaches the card reader from far to near), the step S2 may includefollowing states.

a: When 0<S1<the turn-on voltage of the first inverting sub-module andthe second inverting sub-module, both the logic signal S2 and the logicsignal S2′ may be the high level “1”, and both the first discharge pathand the second discharge path may maintain the turned-off state.

b: When the turn-on voltage of the first inverting sub-module and thesecond inverting sub-module≤S1<the sum of the threshold voltage and theturn-on voltage of the third inverting sub-module and the fourthinverting sub-module, the logic signal S2 may be the low level “0”, thelogic signal S2′ may be the high level “1”, the first discharge path maybe in the turned-on state to maintain discharging, and the seconddischarge path may maintain the turned-off state.

c: When the sum of the threshold voltage and the turn-on voltage of thethird inverting sub-module and the fourth inverting sub-module≤S1, boththe logic signal S2 and the logic signal S2′ may be the low level “0”,and both the first discharge path and the second discharge path may beturned on to maintain discharging.

When the field strength changes from strong to weak (in other words, thechip moves away from the card reader), the step S2 may include followingstates.

a′: When the sum of the threshold voltage and the turn-on voltage of thethird inverting sub-module and the fourth inverting sub-module≤S1, boththe logic signal S2 and the logic signal S2′ may be the low level “0”,and both the first discharge path and the second discharge path may beturned on to maintain discharging.

b′: When the turn-on voltage of the first inverting sub-module and thesecond inverting sub-module≤S1<the sum of the threshold voltage and theturn-on voltage of the third inverting sub-module and the fourthinverting sub-module, the logic signal S2 may be the low level “0”, thelogic signal S2′ may be the high level “1”, the first discharge path maybe in the turned-on state to maintain discharging, and the seconddischarge path may maintain the turned-off state.

c′: When 0<S1<the turn-on voltage of the first inverting sub-module andthe second inverting sub-module, both the logic signal S2 and the logicsignal S2′ may be the high level “1”, and both the first discharge pathand the second discharge path may maintain the turned-off state.

1. A positive feedback latch amplitude limiting control circuit of apassive radio frequency identification (RFID) tag, the circuitcomprising: a signal generating circuit, a signal processing circuit anda discharge circuit, wherein: the signal generating circuit is connectedbetween an output terminal of a rectifier circuit and ground, and isconfigured to: generate a first control signal S1 that varies withchange of strength of a magnetic field coupled to a coil of the passiveRFID tag, input the first control signal S1 to the signal processingcircuit and the discharge circuit, and receive a logic signal S2returned by the signal processing circuit, the signal processing circuitis connected between the output terminal of the rectifier circuit andthe ground, and is configured to: receive the first control signal S1,latch the first control signal S1 to a high level or a low levelaccording to a voltage amplitude of the first control signal S1, obtainthe processed logic signal S2, and return the logic signal S2 to thesignal generating circuit, and the discharge circuit includes a MOStransistor whose gate voltage is indirectly controlled by a latch, andsource and drain of the MOS transistor are respectively connectedbetween the output terminal of the rectifier circuit and the ground,wherein: when the strength of the magnetic field coupled to the coil ofthe passive RFID tag is less than a preset value, the first controlsignal S1 is pulled down to the ground to maintain the discharge circuitin a turned-off state; when the strength of the magnetic field coupledto the coil of the passive RFID tag is greater than the preset value,the first control signal S1 is set to a high level to turn on thedischarge circuit, and charges at the output terminal of the rectifiercircuit are outputted to the ground; and an internal positive feedbacklatch mechanism of the latch makes the first control signal S1 have asubstantially strong pull-down driving force when being pulled to a lowlevel, such that a control gate of the discharge path is fully turnedoff, which avoids a discharge state of a sub-threshold region, andeffectively reduces a leakage current in the discharge path, therebyachieving purposes of saving system power consumption and improvingcommunication performance.
 2. The positive feedback latch amplitudelimiting control circuit of the RFID tag according to claim 1, whereinthe signal generating circuit includes a first current source I1, afirst resistive device and a second resistive device that are connectedin series between a power supply terminal and the ground, wherein: thefirst control signal S1 is outputted from a node between the firstcurrent source I1 and the first resistive device, a gate of a firstN-type MOS transistor is connected to an output terminal of the signalprocessing circuit, a drain thereof is connected to a node between thefirst resistive device and the second resistive device, and a sourcethereof is connected to a node between the second resistive device andthe ground; and a resistance value of the first resistive device is muchsmaller than a resistance value of the second resistive device; and thedischarge circuit includes a MOS transistor whose gate is controlled,and source and drain thereof are respectively connected between thepower supply terminal and the ground, wherein: when the strength of themagnetic field coupled to the coil of the passive RFID tag is less thanthe preset value, the first control signal S1 is pulled down to theground to maintain the discharge circuit in the turned-off state; andwhen the strength of the magnetic field coupled to the coil of thepassive RFID tag is greater than the preset value, the first controlsignal S1 is set to the high level to turn on the discharge circuit, andcharges at the output terminal of the rectifier circuit are outputted tothe ground.
 3. The positive feedback latch amplitude limiting controlcircuit of the RFID tag according to claim 1, wherein: the signalprocessing circuit includes a capacitive device connected to the ground,a first inverting sub-module INV1 and a second inverting sub-moduleINV2, wherein: a positive terminal of the capacitive device is connectedto a latch flip-flop with positive feedback characteristic formed byconnecting an output terminal of the first inverting sub-module INV1 andan input terminal of the second inverting sub-module INV2, andconnecting an input terminal of the first inverting sub-module INV1 andan output terminal of the second inverting sub-module INV2; and thesignal processing circuit converts an analog signal to a digital signal,and is configured to: rapidly convert the first control signal S1 into ahigh-level or low-level logic signal according to an analog voltageamplitude of the first control signal S1, obtain the processed logicsignal S2, and return the logic signal S2 to the signal generatingcircuit.
 4. The positive feedback latch amplitude limiting controlcircuit of the RFID tag according to claim 1, wherein: the signalprocessing circuit includes a detector circuit formed by at least oneunidirectional conduction threshold device and a capacitive device, afirst inverting sub-module INV1 and a second inverting sub-module INV2,wherein: an output terminal of the detector circuit is connected to alatch flip-flop with positive feedback characteristic formed byconnecting an output terminal of the first inverting sub-module INV1 andan input terminal of the second inverting sub-module INV2, andconnecting an input terminal of the first inverting sub-module INV1 andan output terminal of the second inverting sub-module INV2; and thesignal processing circuit converts an analog signal to a digital signal,and is configured to: rapidly convert the first control signal S1 into ahigh-level or low-level logic signal according to an analog voltageamplitude of the first control signal S1, obtain the processed logicsignal S2, and return the logic signal S2 to the signal generatingcircuit.
 5. A positive feedback latch amplitude limiting control circuitof a passive radio frequency identification (RFID) tag, the circuitcomprising a signal generating circuit, a first signal processingcircuit, a first discharge circuit, a second signal processing circuit,a second signal control circuit and a second discharge circuit, wherein:the signal generating circuit is connected between an output terminal ofa rectifier circuit and ground, and is configured to: generate a firstcontrol signal S1 that varies with change of strength of a magneticfield coupled to a coil of the passive RFID tag, input the first controlsignal S1 to the first signal processing circuit, the first dischargecircuit, and the second signal processing circuit, and receive a logicsignal S2 returned by the first signal processing circuit; the firstsignal processing circuit is connected between the output terminal ofthe rectifier circuit and the ground, and is configured to: receive thefirst control signal S1, latch the first control signal S1 to a highlevel or a low level according to a voltage amplitude of the firstcontrol signal S1, obtain the processed logic signal S2, and return thelogic signal S2 to the signal generating circuit; the first dischargecircuit includes a MOS transistor whose gate voltage is indirectlycontrolled by a latch, and source and drain thereof are respectivelyconnected between the output terminal of the rectifier circuit and theground, wherein: when the strength of the magnetic field coupled to thecoil of the passive RFID tag is less than a preset value, the firstcontrol signal S1 is pulled down to the ground to maintain the firstdischarge circuit in a turned-off state; when the strength of thecoupled magnetic field is greater than the preset value, the firstcontrol signal S1 is set to a high level to turn on the dischargecircuit, and charges at the output terminal of the rectifier circuit areoutputted to the ground; and an internal positive feedback latchmechanism of the latch makes the first control signal S1 have asubstantially strong pull-down driving force when being pulled to a lowlevel, such that a control gate of the discharge path is fully turnedoff, which avoids the discharge state of a sub-threshold region; thesecond signal processing circuit is connected between the outputterminal of the rectifier circuit and the ground, and is configured to:receive the first control signal S1, latch the first control signal S1to the high level or the low level according to the voltage amplitude ofthe first control signal S1, obtain a processed logic signal S2′, andinput the logic signal S2′ to the second signal control circuit; thesecond signal control circuit is connected between the output terminalof the rectifier circuit and the ground, and is configured to: generatea second control signal S3 according to the high-level or low-level ofthe logic signal S2′, and input the second control signal S3 to thesecond discharge circuit; the second discharge circuit includes a MOStransistor whose gate voltage is indirectly controlled by the latch, andsource and drain thereof are respectively connected between the outputterminal of the rectifier circuit and the ground, wherein: when thestrength of the magnetic field coupled to the coil of the passive RFIDtag is less than a preset value, the first control signal S1 is pulleddown to the ground to maintain the discharge circuit in the turned-offstate; when the strength of the coupled magnetic field is greater thanthe preset value, the first control signal S1 is set to a high level toturn on the discharge circuit, and the charges at the output terminal ofthe rectifier circuit are outputted to the ground; and an internalpositive feedback latch mechanism of the latch makes the first controlsignal S1 have a substantially strong pull-down driving force when beingpulled to the low level, such that a control gate of the discharge pathis fully turned off, which avoids the discharge state of thesub-threshold region; and the first signal processing circuit includes acapacitive device connected to the ground, a first inverting sub-moduleINV1 and a second inverting sub-module INV2, wherein a positive terminalof the capacitive device is connected to a latch flip-flop with positivefeedback characteristic formed by connecting an output terminal of thefirst inverting sub-module INV1 and an input terminal of the secondinverting sub-module INV2, and connecting an input terminal of the firstinverting sub-module INV1 and an output terminal of the second invertingsub-module INV2; and the second signal processing circuit includes adetector circuit formed by at least one unidirectional conductionthreshold device and a capacitive device, a third inverting sub-moduleINV3 and a fourth inverting sub-module INV4, wherein an output terminalof the detector circuit is connected to a latch flip-flop with positivefeedback characteristic formed by connecting an output terminal of thethird inverting sub-module INV3 and an input terminal of the fourthinverting sub-module INV4, and connecting an input terminal of the thirdinverting sub-module INV3 and an output terminal of the fourth invertingsub-module INV4.
 6. The positive feedback latch amplitude limitingcontrol circuit of the RFID tag according to claim 5, wherein: thesecond signal control circuit includes a second current source, a thirdresistive device and a fourth resistive device that are connected inseries between the power supply terminal and the ground, wherein: asecond control signal S3 is outputted from a node between the secondcurrent source and the third resistive device; a gate of a third N-typeMOS transistor is connected to an output terminal of the second signalprocessing circuit, a drain thereof is connected to a node between thethird resistive device and the fourth resistive device, and a sourcethereof is connected to a node between the fourth resistive device andthe ground; and a resistance value of the third resistive device is muchsmaller than a resistance value of the fourth resistive device.
 7. Amethod of using the above disclosed positive feedback latch amplitudelimiting control circuit in claim 1 to perform a graded amplitudelimiting control, the method comprising following steps: S1: a resonancecircuit is coupled with an external magnetic field to generate analternating current and inputs the alternating current to the rectifiercircuit, and the rectifier circuit rectifies the alternating currentinto a direct current and outputs the direct current to each circuitmodule including the signal generating circuit, the signal processingcircuit and the discharge circuit; S2: the signal generating circuitgenerates the first control signal S1 that varies with the change of thestrength of the magnetic field coupled to the coil of the passive RFIDtag, and inputs the first control signal S1 to the signal processingcircuit, wherein: when the voltage value of the first control signal S1is too low to drive the inverting sub-module in the signal processingcircuit to be turned on, the logic signal S2 outputted by the signalprocessing circuit is a high level “1”; when the voltage value of thefirst control signal S1 gradually increases to the turn-on voltage ofthe inverting sub-module, the logic signal S2 outputted by the signalprocessing circuit is a low level “0”; and the signal processing circuitreturns the logic signal S2 of the high level “1” or the low level “0”to the signal generating circuit; and S3: when the inputted logic signalS2 is the high level “1”, the first N-type MOS transistor in the signalgenerating circuit is turned on, and the second resistive device isshort-circuited; because the resistance value of the first resistivedevice is far smaller than the resistance value of the second resistivedevice, the first control signal S1 is rapidly pulled down to a lowlevel signal, such that the second N-type MOS transistor in thedischarge circuit is turned off, and the discharge path maintains theturned-off state; when the inputted logic signal S2 is the low level“0”, the first N-type MOS transistor is turned off, and the firstcontrol signal S1 maintains a high-level signal, such that the secondN-type MOS transistor in the discharge circuit is turned on, thedischarge path is switched from the turned-off state to the turned-onstate, and the charges at an antenna terminal are outputted to theground.
 8. A method of using the above disclosed positive feedback latchamplitude limiting control circuit in claim 5 to perform a gradedamplitude limiting control, the method comprising following steps: S1: aresonance circuit is coupled with an external magnetic field to generatean alternating current and inputs the alternating current to therectifier circuit, and the rectifier circuit rectifies the alternatingcurrent into a direct current and outputs the direct current to eachcircuit module including the signal generating circuit, the first signalprocessing circuit, the first discharge circuit, the second signalprocessing circuit, the second signal control circuit and the seconddischarge circuit; S2: the signal generating circuit generates the firstcontrol signal S1 that varies with the change of the strength of themagnetic field coupled to the coil of the passive RFID tag, and inputsthe first control signal S1 to the first signal processing circuit andthe second signal processing circuit, wherein: when the voltage value ofthe first control signal S1 is too low to drive the inverting sub-modulein the first signal processing circuit to be turned on, the logic signalS2 outputted by the first signal processing circuit is a high level “1”;when the voltage value of the first control signal S1 graduallyincreases to the turn-on voltage of the inverting sub-module, the logicsignal S2 outputted by the first signal processing circuit is a lowlevel “0”; the first signal processing circuit returns the logic signalS2 of the high level “1” or the low level “0” to the signal generatingcircuit; and when the voltage value of the first control signal S1 islower than the sum of the threshold voltage of the unidirectionalconduction threshold device and the turn-on voltage of the invertingsub-module in the second signal processing circuit, the logic signal S2′outputted by the second signal processing circuit is the high level “1”;when the voltage value of the first control signal S1 graduallyincreases to the sum of the threshold voltage of the threshold deviceand the turn-on voltage of the inverting sub-module in the second signalprocessing circuit, the logic signal S2′ outputted by the second signalprocessing circuit is the low level “0”; and the second signalprocessing circuit inputs the above logic signal S2′ of the high level“1” or the low level “0” to the second signal control circuit; and S3:when the inputted logic signal S2 is the high level “1”, the firstN-type MOS transistor in the signal generating circuit is turned on, andthe second resistive device is short-circuited; because the resistancevalue of the first resistive device is far smaller than the resistancevalue of the second resistive device, the first control signal S1 israpidly pulled down to a low level signal, such that the second N-typeMOS transistor in the first discharge circuit is turned off, and thefirst discharge path maintains the turned-off state; when the inputtedlogic signal S2 is the low level “0”, the first N-type MOS transistor isturned off, and the first control signal S1 maintains a high-levelsignal, such that the second N-type MOS transistor is turned on, thefirst discharge path is switched from the turned-off state to theturned-on state, and the charges at an antenna terminal are outputted tothe ground; when the inputted logic signal S2′ is the high level “1”,the third N-type MOS transistor in the second signal control circuit isturned on, and the fourth resistive device is short-circuited; becausethe resistance value of the third resistive device is far smaller thanthe resistance value of the fourth resistive device, the second controlsignal S3 is rapidly pulled down to a low level signal, such that thefourth N-type MOS transistor in the second discharge circuit is turnedoff, and the second discharge path maintains the turned-off state; whenthe inputted logic signal S2′ is the low level “0”, the third N-type MOStransistor is turned off, and the third control signal S3 maintains ahigh-level signal, such that the fourth N-type MOS transistor is turnedon, the second discharge path is switched from the turned-off state tothe turned-on state, and the charges at the antenna terminal areoutputted to the ground.
 9. The positive feedback latch amplitudelimiting control method according to claim 8, wherein when the fieldstrength changes from weak to strong, the step S2 includes followingstates: a: when 0<S1<the turn-on voltage of the first invertingsub-module and the second inverting sub-module, both the logic signal S2and the logic signal S2′ are the high level “1”, and both the firstdischarge path and the second discharge path maintain the turned-offstate; b: when the turn-on voltage of the first inverting sub-module andthe second inverting sub-module≤S1<the sum of the threshold voltage andthe turn-on voltage of the third inverting sub-module and the fourthinverting sub-module, the logic signal S2 is the low level “0”, thelogic signal S2′ is the high level “1”, the first discharge path is inthe turned-on state to maintain discharging, and the second dischargepath maintains the turned-off state; and c: when the sum of thethreshold voltage and the turn-on voltage of the third invertingsub-module and the fourth inverting sub-module≤S1, both the logic signalS2 and the logic signal S2′ are the low level “0”, and both the firstdischarge path and the second discharge path are in the turned-on stateto maintain discharging; or when the field strength changes from strongto weak, the step S2 includes following states: a′: when the sum of thethreshold voltage and the turn-on voltage of the third invertingsub-module and the fourth inverting sub-module≤S1, both the logic signalS2 and the logic signal S2′ are the low level “0”, and both the firstdischarge path and the second discharge path are in the turned-on stateto maintain discharging; b′: when the turn-on voltage of the firstinverting sub-module and the second inverting sub-module≤S1<the sum ofthe threshold voltage and the turn-on voltage of the third invertingsub-module and the fourth inverting sub-module, the logic signal S2 isthe low level “0”, the logic signal S2′ is the high level “1”, the firstdischarge path is in the turned-on state to maintain discharging, andthe second discharge path maintains the turned-off state; and c′: when0<S1<the turn-on voltage of the first inverting sub-module and thesecond inverting sub-module, both the logic signal S2 and the logicsignal S2′ are the high level “1”, and both the first discharge path andthe second discharge path maintain the turned-off state.